Process for preparing methylhydroxyalkylcellulose having a small number of coloured particles

ABSTRACT

The present invention describes a process for preparing methylhydroxyalkylcellulose having a small number of coloured particles by separation of methylhydroxyalkylcellulose from aqueous suspension with subsequent washing of the methylhydroxyalkylcellulose using a specially equipped rotary pressure filter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit to German Application No. 10 2005 044452 filed Sep. 17, 2005 which is incorporated by reference in itsentirety for all useful purposes.

FIELD OF THE INVENTION

The present invention describes a process for preparingmethylhydroxyalkylcellulose having a small number of coloured particlesby separation of methylhydroxyalkylcellulose from aqueous suspensionwith subsequent washing of the methylhydroxyalkylcellulose using aspecially equipped rotary pressure filter.

BACKGROUND OF THE INVENTION

Methylhydroxyalkylcellulose is generally prepared by alkalization ofpulp and subsequent reaction with etherifying agents such as methylchloride, ethylene oxide or propylene oxide in the presence of suitablesolvents or suspension media (cf. Ullmanns Enzyklopädie der technischenChemie, 4th Edition, Volume 9, page 205, or Houben-Weyl, Methoden derorganischen Chemie, 4th Edition, Volume E 20, part 3, page 2047). Thecrude methylhydroxyalkylcelluloses obtained in the preparation ofmethylhydroxyalkylcellulose contain undesirable by-products such assodium chloride, methanol, glycols, etc. Depending on the degree ofsubstitution, the methylhydroxyalkylcellulose prepared displays adiffering, temperature-dependent dissolution behaviour in aqueous media.The methylhydroxyalkylcelluloses described here are generally insolublein hot water and flocculate out, so that the undesirable by-productsare, according to the prior art, dissolved by suspension of the crudemethylhydroxyalkylcellulose in hot water and are washed out of the crudemethylhydroxyalkylcellulose in a subsequent separation and washing step.When the term methylhydroxyalkylcellulose suspension is used in thefollowing, it is, unless indicated otherwise, always a suspension ofcrude methylhydroxyalkylcellulose and the by-products obtained in thepreparation of crude methylhydroxyalkylcellulose in hot water. Some ofthe separation, purification and washing methods used according to theprior art are described in EP 03 05 898 A2, EP 03 05 899 A2 and EP 0 632056 B1. Depending on the field of application of themethylhydroxyalkylcellulose, it has to meet more or less strict purityrequirements, so that a suitable washing step has to be an indispensablepart of the preparative process.

To be able to prepare products for a wide variety of applications, ithas to be possible to prepare methylhydroxyalkylcelluloses havingdifferent degrees of substitution. The alkyl substitution is generallydescribed by the DS in cellulose ether chemistry. The DS is the meannumber of substituted OH groups per anhydroglucose unit. The methylsubstitution is reported, for example, as DS (methyl) or DS (M). Thehydroxyalkyl substitution is usually described by the MS. The MS is themean number of moles of etherifying reagent bound via an ether bond permole of anhydroglucose unit. The etherification with the etherifyingreagent ethylene oxide is reported, for example, as MS (hydroxyethyl) orMS (HE). The etherification with the etherifying reagent propylene oxideis correspondingly reported as MS (hydroxypropyl) or MS (HP). Thedetermination of the side groups is carried out by the Zeisel method(reference: G. Bartelmus and R. Ketterer, Z. Anal. Chem. 286 (1977)161-190).

The degree of substitution of the methylhydroxyalkylcellulose has aninfluence on the required temperatures of the hot water suspension andthe washing liquids used in the washing step. The temperatures of thehot water suspension and the washing liquids are usually in the rangefrom 80° C. to 100° C., preferably from 85° C. to 98° C.

Various properties of the methylhydroxyalkylcellulose, e.g. the thermalflocculation point, the solubility in water or organic solvents, theviscosity of the solutions produced, the film formation capability, thewater retention capability or the adhesive strength, are set via thedegree of etherification and the type of substituents.

Owing to these many excellent properties, methylhydroxyalkylcellulosesare used in a wide variety of applications.

Firstly, they are used in industrial applications as, for example,thickeners, adhesives, binders, dispersants, water retention agents,protective colloids, stabilizers, suspension media, emulsifiers, filmformation agents and as consistency regulators and processing aids inmineral and dispersion-based building materials systems.

Secondly, they are used in sensitive applications, for example in theproduction of cosmetic and pharmaceutical preparations, in the coatingof tablets, in eye drop suspensions, in contact lens cleaners and thelike or in the production of foodstuffs in which cellulose ethers havinga particularly high purity are required (cf. WO 00/32637). High puritymeans adherence to the appropriate pharmaceutical standards (e.g.pharmaceutical monographs), for example in respect of degree ofsubstitution, heavy metal content, viscosity of the solution, organicimpurities, etc. Furthermore, further legal regulations and additionalwide-ranging customer requirements have to be met.

Thirdly, methylhydroxyalkylcelluloses are used in high-value industrialapplications, for example in the production of speciality ceramics andin suspension polymerization, which likewise require a particularly highpurity. In these applications, high purity means, in particular, lowcontamination of a physical type.

An important quality criterion for customers in the sensitiveapplications and high-value industrial applications is visual assessmentof the methylhydroxyalkylcellulose in respect of physical contamination,known as coloured particles.

Neither pharmaceutical nor other regulations gives specifications inrespect of coloured particles in methylhydroxyalkylcellulose forsensitive applications and high-value industrial applications. Theassessment of this quality criterion has to be undertaken by customersand is generally carried out by means of subjective, visual assessment.Accordingly, the assessment of methylhydroxyalkylcellulose in respect ofcoloured particles is carried out by comparison with samples which areassessed after provision of samples by the customer.

The following four classifications are obtained:

-   -   (1) very good, i.e. better than a comparative sample provided by        the customer    -   (2) good, comparable with a comparative sample provided by the        customer    -   (3) acceptable, only slightly worse than a comparative sample        provided by the customer and    -   (4) not acceptable, i.e. worse than a comparative sample        provided by the customer.

EP 1 375 523 A1 describes a method of selectingmethylhydroxyalkylcellulose in order to obtain, firstly, improvedproperties in the filtration of aqueous solutions of themethylhydroxyalkylcellulose and, secondly, to obtain films having nodefects from the aqueous solution of the methylhydroxyalkylcellulose.The method described here is in principle also suitable for preparingmethylhydroxyalkylcellulose having a low content of coloured particles.However, a disadvantage of the method described in EP 1 375 523 A1 isobviously that selection in industrial production processes always leadsto the inevitable formation of amounts of product which do not meet theselection criteria and accordingly have to be disposed of, subjected toa reprocessing step or processed further in another way, which costsmoney.

After the crude methylhydroxyalkylcellulose has been suspended in hotwater, the methylhydroxyalkylcellulose is separated off from thesuspension medium by methods of the prior art, generally by pressing,centrifugation or filtration, with a distinction being made betweendiscontinuously and continuously operating separation techniques(Grundoperationen chemischer Verfahrenstechnik, 11th Edition, page 164).

In principle, continuously and discontinuously operating pressingapparatuses, e.g. multiplaten presses, strainer presses, plate filterpresses, travelling screen presses and screw presses, are suitable forseparating off the methylhydroxyalkylcellulose from the suspensionmedium. Centrifuges are likewise suitable for separating offmethylhydroxycellulose from the suspension medium. Known types ofcentrifuge include, for example, turnout centrifuges, peelercentrifuges, pusher centrifuges, mesh screw centrifuges, vibratingcentrifuges and sliding centrifuges and decanters.

A further method which has attained importance for separating offmethylhydroxyalkylcellulose is filtration. Here, a distinction islikewise made between discontinuous and continuous apparatuses.Discontinuously operating filters include, for example, fixed-bedfilters, suction filters, candle filters, leaf filters and platefilters. The separation of methylhydroxyalkylcellulose from thesuspension medium by means of discontinuously operating filters isgenerally less preferred. A disadvantage here is the loading andunloading of the filter, which requires a considerable time. Thus,discontinuously operating filters are not suitable for sufficientlylarge, economical throughputs.

Continuously operating filters such as belt filters and rotary filtershave been found to be most useful as separation apparatuses, and rotarypressure filters as are known from WO 02/100512 A1 are particularlysuitable.

These rotary pressure filters allow methylhydroxyalkylcelluloses whichare used, for example, in industrial applications to be separated offeconomically on an industrial scale. However, if themethylhydroxyalkylcellulose is to be used in sensitive applications orhigh-value industrial applications which require a high purity, then theconventional rotary pressure filters optimized for the industrialapplications and the corresponding materials which come into contactwith the suspension and thus with the methylhydroxyalkylcellulose arenot suitable.

It has been found that the use of conventional rotary pressure filtersin the preparation of methylhydroxyalkylcellulose for sensitiveapplications and high-value industrial applications leads to anunacceptable number of coloured particles in themethylhydroxyalkylcellulose. The colour of the particles can, dependingon the rotary pressure filter used, vary from greyish through brownishthrough to black colours.

One considerable way in which these coloured particles can be formed isassociated with the unavoidable abrasion of the plastics used. Thisabraded material leads to no impairment of the product in thepreparation of methylhydroxyalkylcellulose for industrial applications.

In the production process for methylhydroxyalkylcellulose for sensitiveapplications and high-value industrial applications, the suspensions ofthe crude methylhydroxyalkylcellulose and the associated by-products areobtained in particular compositions which display an increased degree ofabrasion toward plastics which are usually employed in rotary pressurefilter technology in the preparation of methylhydroxyalkylcellulose.

This increased abrasive action of the suspensions obtained in thepreparation of methylhydroxyalkylcellulose for sensitive and high-valueindustrial applications may lead to an increased, unacceptable number ofcoloured particles in the end product.

SUMMARY OF THE INVENTION

The invention relates to a rotary pressure filter which comprises aplurality of zones separated from one another by separation plates,including at least one drying zone characterized in that the separationplates which seal off the drying zone(s) comprise heat-resistant,abrasion-resistant plastic and in that the separation plates which sealoff the other zones comprise plastics having good sliding and abrasionproperties.

It was therefore an object of the present invention to provide aneffective and economical, continuous process having high availabilityand a high throughput for the separation and washing of suspensions ofmethylhydroxyalkylcellulose which is insoluble in hot water and theabove-described by-products in hot water so that the end product can,firstly, be prepared in a targeted manner and does not have to beselected from a large amount of product and, secondly, contains only asmall number of coloured particles which is acceptable for sensitive andhigh-value industrial applications.

According to the invention, this object is achieved for the process ofthe abovementioned type by a continuously operating rotary pressurefilter equipped in a specific way with separation plates being used asmeans for separating off the suspension liquid and for washing themethylhydroxyalkylcellulose.

DETAILED DESCRIPTION OF THE INVENTION

Rotary pressure filters are known in industry. A rotary pressure filteris a continuously operating filter having a pressure-tight design. Itconsists essentially of a metallic filter drum which rotates at acontinuously regulatable speed, an associated control head and ametallic, pressure-rated housing. The annular space between the filterdrum and the housing is sealed at the sides by means of stuffing boxesor other sealing systems. The housing is divided radially intopressure-tight chambers by means of zone separators which are pressedpneumatically onto the drum. The surface of the drum comprisesindividual filter cells which are connected via outlet tubes to thecontrol head. A detailed description may be found in WO 02/100512 A1.The suspension to be filtered is fed continually under a constantadmission pressure into the separation zone of the rotary pressurefilter, resulting in a filtercake being built up in the filter cells ofthe rotating drum and subsequently being conveyed into the subsequentchambers for after-treatment, e.g. washing and/or treatment with steam.The filtercake is taken off in an unpressurized zone of the filtereither by means of an automatically operating, adjustable, mechanicalscraper or/and by means of a targeted reverse pulse, typically ofcompressed air, nitrogen or steam. A precise description of the zoneseparators may be found, for example, in WO 02/100512 A1.

In the separation and washing of methylhydroxyalkylcellulose suspensionsby means of rotary pressure filters, the filtercake is treated withsteam according to the prior art. In particular, the filtercake istreated with steam to adjust the residual moisture in a separateseparation zone, known as the drying zone, before the unpressurizedofftake zone. In previously mentioned processes, it is usual to usesaturated steam having pressures of from 0.1 bar gauge to 6.0 bar gauge,preferably from 0.5 bar gauge to 4.5 bar gauge and particularlypreferably from 0.8 bar gauge to 3.0 bar gauge. The pressures mentionedare, according to physical laws, unambiguously associated with thefollowing saturated steam temperatures: 102.3° C. (0.1 bar gauge),111.4° C. (0.5 bar gauge), 116.9° C. (0.8 bar gauge), 143.6° C. (3.0 bargauge), 155.5° C. (4.5 bar gauge), 165.0° C. (6.0 bar gauge) (cf., forexample, VDI-Wärmeatlas).

Furthermore, the present invention concerns an individual part of thezone separator, namely a sealing layer, here referred to as separationplate, which has to be made of appropriately suitable plastics. Thisseparation plate has to meet particular requirements since it is,firstly, fixed rigidly to the housing of the rotary pressure filter andis pressed against the metallic drum and consequently rubs, during therotary motion of the drum, against the ribs which form the individualchambers on the surface of the drum and, secondly, comes into directcontact with the methylhydroxyalkylcellulose suspension to be filteredand washed and accordingly also with the filtercake.

The separation plates of rotary pressure filters are typically made ofthe plastics such as, but not limited to polyethylene (PE),polytetrafluoroethylene (PTFE), if appropriate with addition of, forexample, 40% of, for example, glass fibres, mica, carbon and/orgraphite, polyvinylidene fluoride (PVDF) and polyether ether ketone(PEEK). According to the prior art, the plastic suitable for therespective application is selected and all separation plates of therotary pressure filter are manufactured therefrom. However, thegenerally known mechanical and thermal properties of these plastics canbe employed only to a limited extent as an aid to the choice of thesuitable plastic for the separation elements of a rotary pressure filterwhich is to be used for the separation and washing of a hot watersuspension of crude methylhydroxyalkylcellulose and the associatedby-products as part of the preparation of a methylhydroxyalkylcellulosefor sensitive and high-value industrial applications, since the filtrateto be separated from the methylhydroxyalkylcellulose contains variousby-products, e.g. NaCl in a high concentration, and has a temperaturegreater than 75° C., preferably greater than 85° C. and particularlypreferably greater than 92° C. Both factors have a considerableinfluence on the properties of the plastics in the frictional use asseparating element on the metallic drum of a rotary pressure filter.Thus, the corresponding system “plastic, steel,methylhydroxyalkylcellulose suspension including the by-productspresent” always has to be taken into consideration in the assessment ofthe properties, in particular the resistance, abrasion and frictionalproperties. In the abovementioned process for the separation and washingof methylhydroxyalkylcellulose suspensions for sensitive and high-valueindustrial applications, all plastics which are generally used for theseparation plates have considerable disadvantages in respect ofeconomical use, the resistance under the abovementioned processconditions or in respect of the abrasion resistance and thus the numberof coloured particles in the end product.

Typical and customary materials for the production of such separationplates are, for example, polyethylene (PE), polytetrafluoroethylene(PTFE), polytetrafluoroethylene (PTFE) with addition of typically from10% to 40% of glass fibres, mica, carbon and/or graphite and polyetherether ketone (PEEK).

In the conventional design of the rotary pressure filter in which allseparation plates are made of one of the materials mentioned (i.e. allseparation plates are made of the same material), the number of colouredparticles found in the end product in the preparation ofmethylhydroxyalkylcellulose is unacceptable for sensitive and high-valueindustrial applications.

Surprisingly, we have found that the above-described disadvantages donot occur when a rotary pressure filter is equipped with separationplates which comprise the above-described plastics but the individualseparation plates are made of different plastics.

In this design according to the invention, the plastics of theseparation plates are to be selected so that the separation plates whichseal off the drying zone at both sides comprise a heat-resistant,abrasion-resistant plastic, preferably polyether ether ketone (PEEK),and the remaining separation plates comprise plastics having goodsliding and abrasion properties, preferably polyvinylidene fluoride(PVDF).

When a rotary pressure filter equipped with such separation platesaccording to the invention is used, all the abovementioned disadvantagesin the use of other apparatuses or the use of a rotary pressure filterhaving separation plates made of other plastics are avoided!

Accordingly, the present invention firstly provides a rotary pressurefilter which has a plurality of zones, including a drying zone, whichare separated from one another by separation plates, characterized inthat the separation plates which seal off the drying zone on both sidescomprise heat-resistant (above 145° C., preferably above 165° C.),abrasion-resistant (coefficient of sliding friction against steel, dry,of from 0.5 to 0.31, preferably from 0.4 to 0.31) plastic, particularlypreferably polyether ether ketone (PEEK) and in that the remainingseparation plates comprise one or more plastics having good sliding andabrasion properties (coefficient of sliding friction against steel, dry,of less than 0.4, preferably less than 0.35) and appropriate heatresistance (up to 140° C.), particularly preferably polyvinylidenefluoride (PVDF).

The present invention further provides a process for preparingmethylhydroxyalkylcellulose which comprises filtering and/or washing amethylhydroxyalkylcellulose suspension using a rotary pressure filteraccording to the invention.

In a preferred embodiment, the methylhydroxyalkylcellulose is amethylhydroxypropylcellulose (MHPC) which has a proportion by mass ofhydroxypropoxy groups of from 3.0 to 35.0%, in particular from 4.0 to32.0%, and a proportion by mass of methoxyl groups of from 15.0 to35.0%, in particular from 16.5 to 30.0%.

In another preferred embodiment, the methylhydroxyalkylcellulose is amethylhydroxyethylcellulose (MHEC) which has a degree of substitution MS(HE) of from 0.10 to 1.00, in particular from 0.15 to 0.80, and a degreeof substitution DS (M) of from 1.25 to 2.20, in particular from 1.40 to2.00.

The invention is illustrated by the following examples, without beingrestricted thereto.

EXAMPLES Product Example 1

Hypromellose according to Pharma Monographie as Type 2910: the crudemethylhydroxypropylcellulose to be separated and washed is prepared bythe process described in Houben-Weyl, Methoden der Organischen Chemie,Makromolekulare Stoffe, 4th Edition, Volume E 20, p. 2042 (1987). Afterremoval of excess amounts of reactants, the crudemethylhydroxypropylcellulose obtained had the following properties: DS(M): 1.95 MS (HP): 0.31 Sodium chloride content: 20%

Product Example 2

Hypromellose according to Pharma Monographie as Type 2208: the crudemethylhydroxypropylcellulose to be separated and washed is prepared bythe process described. After removal of excess amounts of reactants, thecrude methylhydroxypropylcellulose obtained had the followingproperties: DS (M): 1.47 MS (HP): 0.21 Sodium chloride content: 16%

Product Example 3

The crude methylhydroxyethylcellulose to be separated and washed isprepared by the process described. After removal of excess amounts ofreactants, the crude methylhydroxyethylcellulose obtained had thefollowing properties: DS (M): 1.58 MS (EP): 0.29 Sodium chloridecontent: 18%

Comparative Example 1

The crude methylhydroxypropylcellulose (product Example 1 or productExample 2) or crude methylhydroxyethylcellulose (product Example 3)suspended in hot water is separated and washed using the above-describedrotary pressure filter. The rotary pressure filter is in this caseequipped with separation plates made of polytetrafluoroethylene (PTFE)with addition of 25% of carbon/graphite. After the subsequent drying andmilling processes, the methylhydroxypropylcellulose ormethylhydroxyethylcellulose is characterized as described above inrespect of the proportion of coloured particles. In all cases, anunacceptable proportion of coloured particles was found (classification:4).

Example 2

The crude methylhydroxypropylcellulose (product Example 1 or productExample 2) or crude methylhydroxyethylcellulose (product Example 3)suspended in hot water is separated and washed using the above-describedrotary pressure filter. The rotary pressure filter is in this caseequipped with separation plates so that the separation plates which sealoff the drying zone on both sides comprise polyether ether ketone (PEEK)and the remaining separation plates comprise polyvinylidene fluoride(PVDF).

After the subsequent drying and milling processes, themethylhydroxypropylcellulose or methylhydroxyethylcellulose ischaracterized as described above in respect of the proportion ofcoloured particles. In all cases, a readily acceptable proportion ofcoloured particles was found (classification: 1-2).

All the references described above are incorporated by reference in itsentirety for all useful purposes.

While there is shown and described certain specific structures embodyingthe invention, it will be manifest to those skilled in the art thatvarious modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described.

1. A rotary pressure filter which comprises a plurality of zonesseparated from one another by separation plates, including at least onedrying zone characterized in that the separation plates which seal offthe drying zone(s) comprise heat-resistant, abrasion-resistant plasticand in that the separation plates which seal off the other zonescomprise plastics having good sliding and abrasion properties.
 2. Therotary pressure filter according to claim 1, wherein the plastic of theseparation plates which seal off the drying zone(s) is heat resistantabove 145° C., and has a coefficient of sliding friction against steel,dry, of from 0.5 to 0.31.
 3. The rotary pressure filter according toclaim 1, wherein the plastic of the separation plates which seal off thedrying zone(s) is heat resistant above 165° C., and has a coefficient ofsliding friction against steel, dry, of from 0.5 to 0.31.
 4. The rotarypressure filter according to claim 1, wherein the plastic of theseparation plates which seal off the other zones is heat-resistant up to140° C. and has a coefficient of sliding friction against steel, dry, ofless than 0.4.
 5. The rotary pressure filter according to claim 3,wherein the plastic of the separation plates which seal off the otherzones is heat-resistant up to 140° C. and has a coefficient of slidingfriction against steel, dry, of less than 0.35.
 6. The rotary pressurefilter according to claim 1, wherein the separation plates which sealoff the drying zone(s) comprise polyether ether ketone (PEEK).
 7. Therotary pressure filter according to claim 5, wherein the separationplates which seal off the drying zone(s) comprise polyether ether ketone(PEEK).
 8. The rotary pressure filter according to claim 1, wherein theseparation plates which seal off the other zones comprise polyvinylidenefluoride (PVDF).
 9. The rotary pressure filter according to claim 7,wherein the separation plates which seal off the other zones comprisepolyvinylidene fluoride (PVDF).
 10. A process for preparingmethylhydroxyalkylcellulose, which comprises the separation of themethylhydroxyalkylcellulose from an aqueous suspension using a rotarypressure filter according to claim
 1. 11. The process according to claim10, wherein the methylhydroxyalkylcellulose is amethylhydroxyethylcellulose (MHEC) or a methylhydroxypropylcellulose(MHPC).
 12. The process according to claim 10, wherein themethylhydroxyalkylcellulose is a methylhydroxypropylcellulose (MHPC)which has a proportion by mass of hydroxypropoxy groups of from 3.0 to35.0% and a proportion by mass of methoxyl groups of from 15.0 to 35.0.13. The process according to claim 10, wherein themethylhydroxyalkylcellulose is a methylhydroxypropylcellulose (MHPC)which has a proportion by mass of hydroxypropoxy groups of from 4.0 to32.0% and a proportion by mass of methoxyl groups of from 16.5 to 30.0%.14. The process according to claim 10, wherein themethylhydroxyalkylcellulose is a methylhydroxyethylcellulose (MHEC)which has a degree of substitution MS (HE) of from 0.10 to 1.00 and adegree of substitution DS (M) of from 1.25 to 2.20.
 15. The processaccording to claim 10, wherein the methylhydroxyalkylcellulose is amethylhydroxyethylcellulose (MHEC) which has a degree of substitution MS(HE) of from 0.15 to 0.80, and a degree of substitution DS (M) of from1.40 to 2.00.